There is a variety of accelerometers available for use in inertial navigation. A typical device is a force feedback accelerometer using, for example, a coil and magnetic field for providing the force feedback and a variable capacitance for sensing deflection to provide an input to the force feedback system. A device of this sort is composed of many parts made from different materials and is consequently expensive to make, difficult to miniaturise, and prone to differential expansion effects which will adversely affect its performance.
These problems can be large overcome by making the accelerometer from a wafer of silicon using the technology of integrated circuit manufacture. There are however other problems to be overcome, in the application of IC technology to accelerometer manufacture. Typically, for integrated circuits, a large number of copies of one circuit are made on one silicon wafer. The wafer is then cut up to form individual circuit chips which are mounted from one face and provided with contact pads on the opposite face.
For many applications of accelerometers the acceleration to be sensed is in an unknown direction, so it is common to use three accelerometers mounted with their sensing axes mutually orthogonal. it would be extremely difficult to mount accurately orthogonal three miniature silicon devices, which are typically only about 0.4 mm thick. Even if this could be achieved, there would be difficulty in making connections to the devices using standard IC equipment since this is designed for mounting and connecting devices having a mounting face and a parallel connection face. Clearly, two accelerometers with orthogonal sensing axes, can be made in the plane of the wafer, but a third would require its sensing axis normal to the wafer.